docs: def treeness(self):
          '''Compute the `treeness` (sum of internal branch lengths / sum of all branch lengths) of this ``Tree``. Branch lengths of ``None`` are considered 0 length
  
          Returns:
              ``float``: `Treeness` of this ``Tree`` (sum of internal branch lengths / sum of all branch lengths)
          '''
          internal = 0.; all = 0.
          for node in self.traverse_preorder():
              if node.edge_length is not None:
                  all += node.edge_length
                  if not node.is_leaf():
                      internal += node.edge_length
          return internal/all